The production of oil and natural gas from an underground well (subterranean formation) can be stimulated by a technique called hydraulic fracturing, in which a viscous fluid composition (fracturing fluid) containing a suspended proppant (e.g., sand, bauxite) is introduced into an oil or gas well via a conduit, such as tubing or casing, at a flow rate and a pressure which create, reopen and/or extend a fracture into the oil- or gas-containing formation. The proppant is carried into the fracture by the fluid composition and prevents closure of the formation after pressure is released. Leak-off of the fluid composition into the formation is limited by the fluid viscosity of the composition. Fluid viscosity also permits suspension of the proppant in the composition during the fracturing operation. Cross-linking agents, such as borates, titanates or zirconates, are usually incorporated into the fluid composition to control viscosity.
Typically, less than one third of available oil is extracted from a well after it has been fractured before production rates decrease to a point at which recovery becomes uneconomical. Enhanced recovery of oil from such subterranean formations frequently involves attempting to displace the remaining crude oil with a driving fluid, e.g., gas, water, brine, steam, polymer solution, foam, or micellar solution. Ideally, such techniques (commonly called flooding techniques) provide a bank of oil of substantial depth being driven into a producing well; however, in practice this is frequently not the case. Oil-bearing strata are usually heterogeneous, some parts of them being more permeable than others. As a consequence, channeling frequently occurs, so that the driving fluid flows preferentially through permeable zones depleted of oil (so-called “thief zones”) rather than through those parts of the strata which contain sufficient oil to make oil-recovery operations profitable.
Difficulties in oil recovery due to thief zones may be corrected by injecting an aqueous solution of an organic polymer and a cross-linking agent into a subterranean formation under conditions where the polymer will be cross-linked to produce a gel, thus reducing permeability of the subterranean formation to the driving fluid (gas, water, etc.). Polysaccharide- or partially hydrolyzed polyacrylamide-based fluids cross-linked with certain aluminum, titanium, zirconium, and boron based compounds are used in these enhanced oil recovery applications. Cross-linked fluids or gels, whether for fracturing a subterranean formation or for reducing permeability of zones in subterranean formation, are now being used in hotter and deeper wells under a variety of temperature and pH conditions. In these operations the rate of cross-linking is critical to the successful generation of viscosity.
Oil field service companies are currently using zirconium or titanium based cross-linkers to generate viscosity in polysaccharide-based fluids useful in hydraulic fracturing, completion and enhanced oil recovery applications. Commercially available, zirconium-based cross-linkers containing an alkanolamine or a hydroxyalkylated ethylenediamine as a chelating ligand cross-link in the desired time and generate and maintain significant viscosity.
For example, U.S. Pat. No. 4,883,605 discloses a water-soluble zirconium chelate formed from a tetraalkyl zirconate and hydroxyethyl-tris-(2-hydroxypropyl) ethylenediamine, and the use of the chelate as a cross-linking agent in hydraulic fracturing fluids and in gels that are used for selectively plugging permeable zones in subterranean formations or for plugging subterranean leaks. Co-pending patent application, “Stable Solutions of Zirconium Hydroxyalkylethylene Diamine Complex and Use in Oil Field Applications”, U.S. patent application Ser. No. 11/643,513, filed Dec. 21, 2006, published at US 2008/0149341 A1, discloses a related complex having a 1:1 molar ratio of zirconium and N,N,N′,N′-tetrakis-(2-hydroxypropyl)-ethylenediamine.
These and other existing titanium or zirconium-based cross-linkers such as TYZOR TE organic titanate or TYZOR TEAZ organic zirconate are liquid products which either freeze or become too viscous to pump under cold temperature conditions, e.g., those encountered in underground wells in Canada and the Rocky Mountains. The need exists for solid cross-linkers which can be used to generate high, thermally stable viscosity in a low and/or high pH environment, and which can be used under these cold temperature conditions by pre-blending with the organic polymer, added as a solid to an aqueous polymer solution or dissolved in water and added to the aqueous polymer solution.
The need also exists for solid cross-linkers in off-shore fracturing operations, where the weight of chemicals being shipped and stored is critical. Solid cross-linkers, which contain two or more times the active Ti or Zr content of their liquid counterparts are desired because they would allow fracturing operations to be completed more economically.
The need also exists for solid cross-linkers that are non-flammable. Existing liquid solvent-based zirconate cross-linkers are flammable liquids. Even aqueous-based liquids generally comprise an organic co-solvent, and therefore are also flammable.
The present invention addresses these needs.